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Unified Dark Matter models aim to describe thedynamical properties of Dark Matter and Dark Energyas different aspects of the same entity which is, inthe simplest case, embodied in a perfect barotropicfluid.Such models, though successful in passingobservational tests on the expansion rate of theUniverse, may present drawbacks in their perturbativestability. Indeed, to be in agreement withobservation, they must possess a sufficiently smallspeed of sound. In this thesis, two counterexamples to this generalconclusion are put forward. First, addressing the generalised Chaplygin gas, onefinds strong hints on the viability of the model alsowhen its asymptotic speed of sound is sufficiently large.Second, a new class of Unified Dark Matter models isintroduced. They are characterised by a fasttransition from a matter-dominated era to a phasewith constant negative pressure. It turns out that, if the transition is fast enough,these models may predict a satisfactory structureformation process and cosmic background radiationanisotropies pattern.